ValidateSchema::evaluate is not thread safe

[saiskee]

Describe the bug

ValidateSchema::evaluate is not thread safe. It is setting member variables of ValidateSchema, in this case m_parserCtx when this variable is used only once in the function. This causes a use-after-free error in the following scenario:

1. T1 runs [xmlSchemaNewParserCtxt](https://github.com/SpiderLabs/ModSecurity/blob/4127c1bf52d2b30a5c2c3e641b8085fd9a720f43/src/operators/validate_schema.cc#L46)
2. T2 runs [xmlSchemaNewParserCtxt](https://github.com/SpiderLabs/ModSecurity/blob/4127c1bf52d2b30a5c2c3e641b8085fd9a720f43/src/operators/validate_schema.cc#L46)
3. T1 runs [freeXmlSchemaParser](https://github.com/SpiderLabs/ModSecurity/blob/4127c1bf52d2b30a5c2c3e641b8085fd9a720f43/src/operators/validate_schema.cc#L130)
4. T2 runs [freeXmlSchemaParser](https://github.com/SpiderLabs/ModSecurity/blob/4127c1bf52d2b30a5c2c3e641b8085fd9a720f43/src/operators/validate_schema.cc#L130), causing a use-after-free on m_parserCtx

Expected behavior/fix
m_parserCtx is no longer needed as a member, so we use a local variable instead.

[martinhsv]

Hi @saiskee ,

The problems with ValidateSchema were first identified (initially as a memory leak) way back with #2469 and addressed in a 3.1 branch: https://github.com/SpiderLabs/ModSecurity/tree/v3/dev/3.1-experimental

It hadn't gotten moved to v3/master so far -- partly simply due to low priority since this operator does not appear to be widely used. I will proceed to do so, however, just to clean it up if nothing else.

Although this item is, in essence, a duplicate, I'll leave it open for now as reminder since the original item was closed with the merge to the other branch.

[martinhsv]

Closing this duplicate now, since the fix that was originally applied to a dev branch has been merged to v3/master via #3005